1. Field of the Invention
The present invention relates to a radiation imaging system for imaging a radiographic image of an object and a control method thereof, and a radiation image detecting device used in the radiation imaging system.
2. Description Related to the Prior Art
In a medical field, an X-ray imaging system using X-rays, for example as a kind of radiation, is known. The X-ray imaging system is constituted of an X-ray generating apparatus for applying the X-rays to an object and an X-ray image detecting device, which receives the X-rays passed through the object and detects an X-ray image representing image information of the object. In the X-ray image detecting device, an X-ray film, an imaging plate (IP), or the like is conventionally used as a detection panel. However, an X-ray image detecting device using a flat panel detector (FPD) as the detection panel has become widespread presently. The FPD has a matrix of pixels each for accumulating signal charge in accordance with the amount of the X-rays incident thereon. The FPD converts the accumulated signal charge into a voltage signal on a pixel-by-pixel basis at its signal processing circuit, and thereby detects the X-ray image representing the image information of the object and outputs the X-ray image as digital image data.
Some X-ray imaging systems have an automatic exposure control (AEC) function in which a photo timer disposed in front of an X-ray imaging apparatus measures a dose of the X-rays passed through the object and issues a stop signal to the X-ray generating apparatus to stop an X-ray emission, as soon as the X-ray dose has reached a predetermined value. There is also known an X-ray image detecting device that carries out the AEC using a part of pixels of the FPD or a dose detection sensor disposed in an image capturing field of the FPD, instead of the photo timer (for example, refer to U.S. Pat. No. 7,078,701 corresponding to Japanese Patent Laid-Open Publication No. 2004-251892). In performing the AEC in the X-ray image detecting device having the integral dose detection sensor, the X-ray image detecting device issues the stop signal to the X-ray generating apparatus. The photo timer itself is an X-ray absorber that absorbs the X-rays of the order of approximately 5%. Thus, using the photo timer in the AEC requires an X-ray dose to be increased by an X-ray absorption amount by the photo timer, and hence causes increase in radiation exposure of the object. Using the X-ray sensor of the FPD, contrarily, does not bring about such a problem.
In the X-ray imaging system using the X-ray image detecting device having the FPD, communication of various types of signals and data is established among the X-ray generating apparatus, the X-ray image detecting device, and a console. The console is a device for setting an imaging condition and displaying the captured X-ray image. The console transmits to the X-ray image detecting device a life check signal for checking whether or not the X-ray image detecting device is activated, a state monitoring signal for making an inquiry about a state such as a temperature to the X-ray image detecting device, a calibration command for commanding the X-ray image detecting device to perform a calibration, and the like. The X-ray image detecting device transmits to the console a response signal responding to each of the above signals, an error signal for making a notification of an error, captured X-ray image data, and the like. The X-ray generating apparatus and the X-ray image detecting device transmit and receive therebetween a synchronization signal for notifying the X-ray image detecting device of a start of an X-ray emission from the X-ray generating apparatus, the stop signal for making the X-ray generating apparatus stop the X-ray emission, a response signal for making a notification of the stop of X-ray emission, and the like.
As described above, the X-ray image detecting device communicates the various types of signals with the console. Furthermore, the synchronization signal is communicated before the start of an X-ray emission between the X-ray image detecting device and the X-ray generating apparatus, in order to synchronize X-ray emission timing by the X-ray generating apparatus and image accumulation timing by the X-ray image detecting device. Additionally, in performing the AEC, the stop signal is communicated between the X-ray image detecting device and the X-ray generating apparatus.
In the communication among the essential devices of the X-ray imaging system, e.g. between the X-ray image detecting device and the console and between the X-ray image detecting device and the X-ray generating apparatus, communications line congestion and signal collision cause the occurrence of a communication delay. The communications line congestion means a case where a plurality of signals heading for the same direction are transmitted at almost the same time through a communications line. The signal collision means a collision of signals transmitted in both directions of the communications line.
As measures against the communication delay in the synchronization signal between the X-ray image detecting device and the X-ray generating apparatus, according to an X-ray imaging system of U.S. Pat. No. 7,856,085 corresponding to Japanese Patent Laid-Open Publication No. 2010-035778, for example, it is judged that whether a communications method between the devices adopts dedicated line communications, wireless communications, or network communications. In the case of the wireless communications or the network communications in which the communication delay tends to occur, FPD drive timing is changed in anticipation of the communication delay.
To be more specific, according to the X-ray imaging system of the U.S. Pat. No. 7,856,085, the X-ray image detecting device issues a preparation completion signal to the X-ray generating apparatus, when being ready for receiving an X-ray emission. Upon receiving the preparation completion signal, the X-ray generating apparatus starts an X-ray emission. An emission time of the X-rays is set in advance, and the X-ray generating apparatus stops the X-ray emission after a lapse of the emission time. In the X-ray image detecting device, the issue of the preparation completion signal triggers a start of an image accumulation operation. An image accumulation time is set longer than the emission time such that the image accumulation operation has been continued during the set emission time. In a case where the communications method is the wireless communications or the network communications excepting the dedicated line communications, the communication of the preparation completion signal is delayed more than that in the case of the dedicated line communications, and thereby the timing of starting the X-ray emission is delayed. The larger the communication delay, the more the timing of starting the X-ray emission is delayed. At the worst, such a situation may occur that the X-ray generating apparatus keeps emitting the X-rays even after the X-ray image detecting device completes the image accumulation operation. To handle such a problem, in the X-ray imaging system of the U.S. Pat. No. 7,856,085, the FPD drive timing is varied in the communications method without using the dedicated line to make the image accumulation time longer than that of the dedicated line communications.
Also, as measures against a communication delay between the X-ray image detecting device and the console, according to an X-ray imaging system of Japanese Patent Laid-Open Publication No. 2010-057525, for example, bidirectional communications between the X-ray image detecting device and the console is withdrawn in order to prevent a delay in communication of imaging order information due to the signal collision, and using unidirectional communications prevents the communication delay.
As described above, the U.S. Pat. No. 7,856,085 describes the measures against the communication delay in the synchronization signal (preparation completion signal) at a time of starting communication between the X-ray image detecting device and the X-ray generating apparatus. The Japanese Patent Laid-Open Publication No. 2010-057525 describes the measures against the communication delay between the X-ray image detecting device and the console.
In carrying out the AEC, as described in the X-ray imaging system of the U.S. Pat. No. 7,078,701, it is required that the stop signal is transmitted without a delay from the X-ray image detecting device to the X-ray generating apparatus. This is because a delay occurring in the stop signal prevents the X-ray emission from stopping at an appropriate time, and therefore brings about increase in the radiation exposure of the object. The delay in stopping the X-ray emission also causes application of the X-rays beyond a target dose, and hence results in deterioration in the quality of the X-ray image.
The methods described in the U.S. Pat. No. 7,856,085 and the Japanese Patent Laid-Open Publication No. 2010-057525 have no consideration for the AEC, and adopting these methods does not become measures directed toward the elimination of the communication delay in the stop signal. The method of the U.S. Pat. No. 7,856,085 relates to measures against a delay in emission start timing due to a delay in synchronization communication at a time of starting the emission. Elongating the image accumulation time in anticipation of the communication delay cannot solve the communication delay in the stop signal, as a matter of course.
In the method of the Japanese Patent Laid-Open Publication No. 2010-057525, the communication method between the console and the X-ray image detecting device is limited to the unidirectional communications from the console to the X-ray image detecting device, for the purpose of preventing the communication delay in transmitting the order information from the console to the X-ray image detecting device. In performing the AEC, the console sometimes mediates the transmission of the stop signal from the X-ray image detecting device to the X-ray generating apparatus. In such a case, the stop signal has to be transmitted from the X-ray image detecting device to the console. Furthermore, the bidirectional communications of the various types of signals is required between the X-ray image detecting device and the console, which includes a transmission of the life check signal for checking an actuation state of the X-ray image detecting device and a response thereof, a transmission of the state monitoring signal for monitoring the state such as the temperature of the X-ray image detecting device and a response thereof, and the like, in addition to a transmission and a response of the order information and the stop signal. Thus, adopting this unidirectional communications is unrealistic.